Studies are planned or are in progress on the mechanism of proton activation in biotin dependent carboxylases and on te chemistry and funtion of free enolpyruvate. We are developing the isotope partition method in which one starts from the labeled ternary complex (ES1S2) and chases with unlabeled substrate to determine the rate of desorbtion of substrate. Previously the method has been used with binary complexes. Carbamyl-P synthetase is being studied by the oxygen scrambling technique to determine if carboxyphosphate might be an intermediate. Isotope effects are also being used to study the mechanism of phosphoryl transfer first in model reactions and later with several enzymes.